The mid-infrared band (3-5 μm) is an important atmospheric transmission window. Laser in this band can transmit through fog, smoke or the like in the atmosphere. Therefore, mid-infrared laser can be widely used in military applications, such as laser guiding, electrooptical countermeasures, object detection, and the like. Further, most hydrocarbon gas and other toxic gas molecules have relatively strong absorption in the 3-5 μm band. Therefore, the mid-infrared laser also has a variety of applications in gas detection, atmospheric remote sensing and environmental protection, and the like.
Due to absence of direct laser gain medium, the mid-infrared laser is generated mostly by nonlinear optical frequency conversion, such as optical parametric oscillation, optical parametric amplification, and difference frequency generation. In the 3-5 μm band, the nonlinear optical crystals used so far include LiNbO3, KTiOPO4, AgGaS2, ZnGeP2, and the like. They usually have relatively high nonlinear optical coefficients, but their laser induced damage thresholds are very low. For example, LiNbO3 has a laser induced damage threshold of about 120 MW/cm2 (1.064 μm, 30 ns), KTiOPO4 has a laser induced damage threshold of about 150 MW/cm2 (1.064 μm, 30 ns), AgGaS2 has a laser induced damage threshold of about 25 MW/cm2 (1.064 μm, 35 ns), and ZnGeP2 has a laser induced damage threshold of about 3 MW/cm2 (1.064 μm, 30 ns) (see Dmitriev et al., Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1999, p. 118). Therefore, it is the low damage threshold that limits the various applications of those mid-infrared nonlinear optical crystals.
Silicon carbides crystalline in more than 250 polytypes, among which, 3C silicon carbide, 4H silicon carbide, and 6H silicon carbide are the most common ones. Especially, 4H silicon carbide and 6H silicon carbide have non-zero 2nd-order nonlinear optical coefficients, and have the following characteristics:    1. Relatively large 2nd-order nonlinear optical coefficients (d15=6.7 pm/V for 4H silicon carbide, and d15=6.6 pm/V for 6H silicon carbide) (see Sato et al., Accurate measurements of second-order nonlinear optical coefficients of 6H and 4H silicon carbide, Journal of the Optical Society of America B 26, 1892 (2009));    2. Relatively high transmittance in the visible and infrared spectra (specifically, 4H silicon carbide is transmissive in a range of 0.38-5.5 μm, and 6H silicon carbide is transmissive in a range of 0.4-5.5 μm);    3. Relatively high laser induced damage thresholds (greater than 80 GW/cm2 (1.064 μm, 10 ns) for both 4H and 6H silicon carbide) (see Niedermeier et al., Second-harmonic generation in silicon carbide polytypes, Applied Physics Letter. 75, 618 (1999));    4. High thermal conductivity (490 Wm−1K−1 for both 4H and 6H silicon carbide), good chemical stability, not easy to deliquescence; and    5. Developed crystal growth technology, and good crystal qualities.
4H and 6H silicon carbide both are positive uniaxial crystals (no<ne). Prior to studying the nonlinear optical properties, it is important to accurately measure refractive indices of them. At a given temperature, the phase matching condition for nonlinear optical frequency conversion is determined only by refractive indeices of the crystal. The nonlinear frequency conversion can be performed efficiently and thus becomes feasible only if the phase matching condition is satisfied.
In 1944, Thibault firstly measured, using the method of minimum deviation angle, the refractive indices of the 6H silicon carbide crystal in a visible band (0.4047-0.6708 μm) with a measurement precision of about 3×10−4 (see Thibault, Morphological and structural crystallography and optical properties of silicon carbide (SiC), The American Mineralogist 29, 327 (1944)). In 1968, Choyke et al. measured, using the Newton interference of equal inclination, the refractive indices for o-ray (no) of the 6H silicon carbide crystal with a measurement precision of about 2×10−3, and extended no to the ultraviolet and infrared bands (see Choyke et al., Refractive index and low-frequency dielectric constant of 6H SiC, Journal of the Optical Society of America 58, 377 (1968)). In 1971, Shaffer measured the refractive indices of the 4H and 6H silicon carbide crystals in a visible band (0.467-0.691 μm) with a measurement precision of about 1×10−3, and obtained their dispersion equations (see Shaffer, Refractive index, dispersion, and birefringence of silicon carbide polytypes, Applied Optics 10, 1034 (1971)).
U.S. Pat. No. 3,676,695 issued in 1972, entitled “Nonlinear optical devices utilizing substantially hexagonal silicon carbide,” and also its patent family (CA 962755, NL 7210039, SE 3676695, IT 964758, GB 1375638, FR 2147103, DE 2235800, and BE 786555) disclosed the refractive indices of a hexagonal silicon carbide crystal at six wavelengths (0.488 μm, 0.5017 μm, 0.5145 μm, 0.5321 μm, 0.6328 μm, and 1.064 μm) measured by the method of minimum deviation angle. The absorption spectrum of the silicon carbide crystal disclosed in this patent shows that the shortest transmissive wavelength for this crystal is 0.4 μm, corresponding to a band gap (3.0 eV) of 6H silicon carbide. The refractive indices further demonstrate that this crystal is 6H silicon carbide. This patent proposed that 6H silicon carbide can be used as a nonlinear optical crystal for frequency conversion, such as frequency multiplication and optical parametric conversion, by angular phase matching, and that at least one light beam participating in the nonlinear optical frequency conversion has a wavelength greater than 1 μm. The inventors of this patent, Singh et al., proposed in a later published paper that 6H silicon carbide can achieve phase matching for frequency conversion when a fundamental light has a wavelength greater than 2 μm, and especially that a phase matching angle for second harmonic generation is about 75° when the fundamental light has a wavelength of 2.128 μm (see Singh et al., Nonlinear optical properties of hexagonal silicon carbide, Applied Physics Letters 19, 53 (1971)). It is to be noted that in this patent a light source adopted in measuring the refractive indices of 6H silicon carbide has a maximal wavelength of 1.064 μm, but the nonlinear optical frequency conversion involves a relatively large wavelength in the infrared band (e.g., 2.128 μm). It is known that the refractive index at a relatively large wavelength obtained by extrapolation from a dispersion equation fitted based on refractive indices at relatively short wavelengths will deviate a lot from the real one. The inventors of the present application demonstrate with new refractive index data that 6H silicon carbide is out of the question for second harmonic generation and optical parametric conversion in the infrared band. In other words, the above patents issued to Singh et al. are impossible to practice, which will be described in more detail in the following.
In 1985, Choyke et al. provided in a document (Choyke et al., Handbook of Optical Constants of Solids, Academic, New York, 1985, p. 593) data relating to the refractive index no of 6H silicon carbide, most of which were cited from the above papers by Thibault in 1944, Choyke et al. in 1968, and Shaffer et al. in 1971, and just simply listed the data from these papers. Because these three works measured the refractive index in different ways, many of the data relating to the refractive index no provided by Choyke et al. in 1985 are not reasonable. According to the well known knowledge, the refractive index no should decrease as the wavelength increases. However, the refractive index no proposed in the document of Choyke et al. does not follow this rule. For example, in this document, no has a value of 2.684 at the wavelength of 0.4959 μm, which is smaller than that (2.687) at the wavelength of 0.498 μm. In 2003, Baugher et al. measured birefringence data (ne-no) of 6H silicon carbide by using the data relating to the refractive index no provided by Choyke et al. in 1985, and pointed out by computations that 6H silicon carbide can satisfy the phase matching condition for optical parametric oscillation (see Baugher et al., Temperature dependence of the birefringence of SiC, Optical Materials 23, 519 (2003)). Baugher et al. measured only the birefringence data of the 6H silicon carbide crystal, and cited the incorrect refractive index data. As a result, their conclusion that the phase matching condition for optical parametric oscillation can be satisfied in 6H silicon carbide is incorrect.
As can be seen from the above, most of the prior art documents or papers measured the refractive indices of the 6H silicon carbide crystal in the visible band, and there are rare results in the infrared band at a relatively longer wavelength side. However, the nonlinear optical frequency conversion of the 6H silicon carbide crystal generally involves the infrared band. To reduce the deviation caused by extrapolation of the refractive index based on from the dispersion equations based on refractive indices of relatively short wavelengths, it is pressing and important to preciously measure the refractive indices of 6H silicon carbide in the infrared band.
The inventors of the present application measured, using the method of minimum deviation angle, the refractive indices (no and ne) of the 6H silicon carbide crystal in both a visible band and an infrared band (0.4358-2.325 μm), with a precision of about 3×10−5, and fitted the dispersion equations for the 6H silicon carbide crystal. Compared to the refractive index data proposed in the above documents, the measurement result by the inventors generally coincides with the refractive index data in those documents in the visible band, but shows relatively great dispersion in the infrared band.
Further, the inventors of the present application computed the phase matching condition for nonlinear frequency conversion of the 6H silicon carbide crystal. 6H silicon carbide crystal has a point group of 6 mm, and only Type II angular phase matching exits therein. For the angular phase matching of second harmonic generation, it should be satisfied that n1o+n1e>2n2o (which is derived from that a sine value of a phase matching angle should be less than 1), where n1o and n1e indicate an o-ray refractive index and an e-ray refractive index for fundamental light, respectively, and n2o indicates an o-ray refractive index for light after second harmonic generation. As 6H silicon carbide has relatively great dispersion while relatively small birefringence in the infrared band, computations show that the 6H silicon carbide crystal cannot achieve the phase matching conditions for second harmonic generation in its transmissive band (0.4-5.5 μm). For nonlinear frequency conversion such as optical parametric conversion and difference frequency generation, the phase matching condition is: n3oω3−n1e(θ)ω1=n2oω2, where ω3 and ω1 indicate frequencies of pumping light, ω2 indicates a frequency of infrared light, n30 indicates an o-ray refractive index for the pumping light ω3, n2o indicates an o-ray refractive index for the infrared light ω2, and n1e(θ) indicates an e-ray refractive index for the pumping light ω1 at an angle θ with respect to the optical axis of crystal. Computations show that the 6H silicon carbide crystal cannot achieve the phase matching conditions for optical parametric conversion or difference frequency generation in its transmissive band. The U.S. patent (U.S. Pat. No. 3,676,695) and the paper of Baugher et al. in 2003 adopted the incorrect refractive index data, and thus arrived at the incorrect conclusion that the 6H silicon carbide crystal can achieve the phase matching for nonlinear frequency conversion in the mid-infrared band.
In 1971, Shaffer measured refractive indices of 4H silicon carbide in a visible band (0.467-0.691 μm) (see Shaffer, Refractive index, dispersion, and birefringence of silicon carbide polytypes, Applied Optics 10, 1034 (1971)). There is no report on nonlinear optical properties of the 4H silicon carbide crystal and applications of the 4H silicon carbide crystal in nonlinear optical devices.